This invention relates to a digital communication system which communicates a sequence of transmission signals from a transmitter (namely, a coder) to a receiver (namely, a decoder) and an error correction method used in the digital communication system
A recent requirement has been directed to promotion of digitalization of communication and has given rise to hot discussion about introducing error correction into an actual system in order to improve quality of communication. Herein, it is known in the art that a very strong error correction system or a data communication system can be structured by combining a convolution coding method with a maximum likelihood decoding method.
Practically, such an error correction method is described by F. J. MacWllliams et al in "The Theory of Error-Correcting Codes" published by North-Holland Publishing Company.
In this event, the convolution coding method in a coder is for producing a sequence of convolution codes in response to a present input bit and a predetermined number of preceding input bits. The convolution codes are given by adding the present and the preceding input bits to modulus 2.
On the other hand, a Viterbi decoding method is used as the maximum likelihood decoding method in a decoder so as to decode the convolution codes given as a sequence of reception codes into a sequence of decoded signals. More specifically, the Viterbi decoding method may be considered to be executed in accordance with a trellis structure which includes a plurality of nodes connected to branches and paths formed by a combination of the branches, as well known in the art. At first, branch metrics between each of the reception codes and candidate codes are calculated at the respective branches to determine correlation values or branch metrics between each of the reception codes and the candidate codes at the respective nodes and are added to survived path metrics to calculate total path metrics at the paths. Subsequently, maximum likelihood sequences are selected at the respective nodes by comparing the total path metrics with one another and are determined to select a survived path and to detect the decoded signals.
According to the Viterbi decoding method, it is possible to correct an error or errors in the reception codes.
Herein, it is to be noted that a great amount of hardware is required in the decoder in order to decode the reception codes into the decoded signals in accordance with the maximum likelihood decision rule, and in particular, the Viterbi decoding method. Consequently, the decoder should have a path memory of a great amount of a capacity memory and becomes inevitably large in structure.
Alternatively, attempts have been also made in the Viberbi decoding method to reduce an amount of hardware and to make a decoder small in size. However, such a decoder has a disadvantage that a possibility of selecting a wrong path becomes high with a reduction of the hardware.